Words for sex and prostitution move easily from language to language. Consider the peregrinations of bordel all over the Romance-speaking world (the noun became “international,” though surely the introduction of the “thing” did not need help from the neighbors), the etymology of ribald (from French, where it is from Germanic: the root (h)rib- meant “copulate”), and the unhealthy popularity of our F-word in the remotest countries of the planet. The OED provides evidence that bordel made its way into Middle English, and in light of this circumstance I would risk defending and developing an etymology offered in The Century Dictionary but disregarded by all later authorities. Unlike breþel, broþel from breoþan — I suspect — never existed. Much more likely, when bordel surfaced in Middle English, it was associated with broþen (the participle). The variation ro ~ or and the like (that is, r preceding a vowel versus r following it: metathesis) is common; thus, the German for board is Brett. Given this scenario, brothel will emerge as a trivial folk etymological alternation of a foreign word. Probably no one noticed that bordel reminded one of board. People needed a vivid picture of a house of sin, not an exercise in historical linguistics.